Acoustic wave device

ABSTRACT

An acoustic wave device includes: a piezoelectric substrate that is made of a single crystal piezoelectric material, and includes a first region including an upper surface, and a second region that is located under the first region and has a density less than a density of the first region; and an IDT located on the upper surface of the piezoelectric substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2016-055379, filed on Mar. 18,2016, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of the present invention relates to an acoustic wavedevice.

BACKGROUND

In acoustic wave devices, an Interdigital Transducer (IDT) exciting anacoustic wave is formed on a piezoelectric substrate. The piezoelectricsubstrate is, for example, a lithium tantalate (LiTaO₃) substrate or alithium niobate (LiNbO₃) substrate. When the Li compositions in lithiumtantalate and lithium niobate are stoichiometric, they are called astoichiometry composition. When the Li composition is a little less thanthe stoichiometric composition, it is called a congruent composition.Most of the lithium tantalate substrates and the lithium niobatesubstrates have a congruent composition.

Japanese Patent Application Publication No. 2013-66032 (PatentDocument 1) describes that lithium is diffused to the surface of asubstrate with a congruent composition to form a region with astoichiometry composition on the substrate surface. Japanese PatentApplication Publication Nos. 2011-135245 and 2002-305426 (PatentDocuments 2 and 3) describe that a piezoelectric substance with astoichiometry composition is used for an acoustic wave device.International Publication No. 2013/031651 (Patent Document 4) describesthat a dielectric film is provided under a piezoelectric film.

To reduce the loss of the acoustic wave device, it is required to reducethe leak of the acoustic wave excited by the IDT. However, there is noknown preferable structure in the piezoelectric substrate for reducingthe loss of the acoustic wave device.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided anacoustic wave device including: a piezoelectric substrate that is madeof a single crystal piezoelectric material, and includes a first regionincluding an upper surface, and a second region that is located underthe first region and has a density less than a density of the firstregion; and an IDT located on the upper surface of the piezoelectricsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of an acoustic wave resonator in accordance witha first embodiment, and FIG. 1B is a cross-sectional view taken alongline A-A in FIG. 1A;

FIG. 2 is a cross-sectional view of a piezoelectric substrateillustrating an image of a leaky waves and a bulk wave in thepiezoelectric substrate;

FIG. 3 is a graph of acoustic velocity versus depth in the piezoelectricsubstrate in the first embodiment;

FIG. 4A through FIG. 4D are cross-sectional views illustrating a methodof fabricating the acoustic wave resonator of the first embodiment;

FIG. 5 is a cross-sectional view of an acoustic wave resonator inaccordance with a second embodiment;

FIG. 6 is a graph of acoustic velocity versus depth in the piezoelectricsubstrate and a support substrate in the second embodiment;

FIG. 7A through FIG. 7C are cross-sectional views illustrating a methodof fabricating the acoustic wave resonator in accordance with the secondembodiment; and

FIG. 8A is a circuit diagram of a ladder-type filter in accordance witha third embodiment, and FIG. 8B is a block diagram of a multiplexer inaccordance with a variation of the third embodiment.

DETAILED DESCRIPTION

A description will be given of embodiments of the present invention withreference to the accompanying drawings.

First Embodiment

An acoustic wave resonator will be described as an acoustic wave device.FIG. 1A is a plan view of an acoustic wave resonator in accordance witha first embodiment, and FIG. 1B is a cross-sectional view taken alongline A-A in FIG. 1A. As illustrated in FIG. 1A and FIG. 1B, an IDT 21and reflectors 22 are formed on a piezoelectric substrate 10. The IDT 21and the reflectors 22 are formed of a metal film 12 formed on thepiezoelectric substrate 10. The IDT 21 includes a pair of comb-shapedelectrodes 20 facing each other. The comb-shaped electrode 20 includes aplurality of electrode fingers 14 and a bus bar 18 to which theelectrode fingers 14 are connected. The pair of comb-shaped electrodes20 are arranged so as to face each other so that the electrode fingers14 of one of the comb-shaped electrodes 20 and the electrode fingers 14of the other are arranged substantially in an alternate order. Theacoustic wave excited by the IDT 21 mainly propagates in the alignmentdirection of the electrode fingers 14. The pitch of the electrodefingers 14 is approximately equal to the wavelength λ of the acousticwave. The piezoelectric substrate 10 is a lithium tantalate substrate ora lithium niobate substrate. The metal film 12 is, for example, analuminum film, a copper film, a titanium film, or a chrome film, or acomposite film of at least two of them. The metal film 12 has a filmthickness of, for example, 100 to 400 nm.

As illustrated in FIG. 1B, the piezoelectric substrate 10 includes afirst region 10 a, a second region 10 b, and a third region 10 c. Thefirst region 10 a includes the upper surface of the piezoelectricsubstrate 10. The IDT 21 and the reflectors 22 are located on the uppersurface of the piezoelectric substrate 10. The second region 10 b islocated under the first region 10 a. The third region 10 c is locatedbetween the first region 10 a and the second region 10 b. The firstregion 10 a is a region having a congruent composition. The secondregion 10 b has a stoichiometry composition. The third region 10 c is atransition region from the congruent composition to the stoichiometrycomposition. In the stoichiometry composition, a composition ratio oflithium to lithium and tantalum (or niobium) (hereinafter, described asa lithium composition ratio) is 49.5% or greater and 50.5% or less. Inthe congruent composition, the lithium composition ratio is 49.5% orless. The lithium composition ratio is, for example, 48% or greater. Ineach of the first region 10 a and the second region 10 b, the lithiumcomposition ratio is substantially constant. In the third region 10 c,the lithium composition ratio gradually changes. The third region 10 cmay not necessarily be located.

For example, in an acoustic wave device using a leaky wave, the acousticwave excited by the IDT 21 is mainly a leaky wave. The IDT 21 emits abulk wave in addition to the surface acoustic wave. Since the bulk wavedoes not contribute to resonance, as the energy of the bulk waveincreases, the loss of the resonator increases.

FIG. 2 is a cross-sectional view of the piezoelectric substrateillustrating an image of a leaky wave and a bulk wave in thepiezoelectric substrate. In FIG. 2, the x1 direction is the propagationdirection of a leaky wave on the surface of the piezoelectric substrate10, the x2 direction is a direction perpendicular to the x1 direction onthe surface of the piezoelectric substrate 10, and the x3 direction isthe depth direction of the piezoelectric substrate 10. The maindisplacement component of a leaky wave is an SH wave. Thus, the leakywave is displaced mainly in the x2 direction. On the other hand, theleaky wave propagates while emitting a bulk wave into the piezoelectricsubstrate 10. The emission of the bulk wave causes the loss of theacoustic wave device.

FIG. 3 is a graph of acoustic velocity versus depth in the piezoelectricsubstrate in the first embodiment. In the following description, theacoustic velocity of the bulk wave will be focused on, but therelationship between the acoustic velocity of the bulk wave and thelithium composition ratio is substantially the same as the relationshipbetween the acoustic velocity of the surface acoustic wave and thelithium composition ratio. Therefore, a description will be simply givenof the acoustic velocity. The acoustic velocity in the stoichiometrycomposition is greater than the acoustic velocity in the congruentcomposition. Thus, as illustrated in FIG. 3, the acoustic velocity inthe first region 10 a is less than the acoustic velocity in the secondregion 10 b. In the third region 10 c, the acoustic velocity graduallychanges. The energy of the acoustic wave concentrates in the region inwhich the acoustic velocity is low. For example, when a leaky wave isused, the boundary between the first region 10 a and the second region10 b is structured to be located substantially between the leaky waveand the bulk wave in FIG. 2. This structure inhibits the emission of abulk wave because the velocity of the bulk wave is fast in a deepregion. Accordingly, the energy concentrates in the first region 10 a.Therefore, the insertion loss of the acoustic wave resonator can beimproved.

The measured acoustic velocity of a Rayleigh wave in a 42° rotated Y-cutX-propagation lithium tantalate substrate by a linear focused beamacoustic microscope is approximately 3125 m/second in the congruentcomposition, and is approximately 3170 m/second in the stoichiometrycomposition. The acoustic velocity of the surface acoustic wave isproportional to the square root of (elastic modulus/density). Theelastic modulus relates to a Young's modulus and a Poisson ratio.Between the stoichiometry composition and the congruent composition, theYoung's moduluses and the Poisson ratios are approximately the same. Incontrast, the density of the congruent composition is greater than thedensity of the stoichiometry composition. For example, in a lithiumtantalate substrate, the density of the congruent composition is 7454kg/m³, while the density of the stoichiometry composition is 7420 to7440 kg/m³. Thus, the acoustic velocity in the stoichiometry compositionis greater than the acoustic velocity in the congruent composition.

In Patent Document 4, located under a lithium niobate substrate is adielectric film such as a silicon oxide film or a silicon nitride film.The silicon oxide film or the silicon nitride film has an acousticvelocity greater than that of lithium niobate. However, in thisstructure, a bulk wave is reflected by a boundary face between thelithium niobate substrate and the dielectric film. As a result, spuriousdue to the bulk wave occurs. On the other hand, the first embodimentprovides the first region 10 a in which the acoustic velocity is highand the second region 10 b in which the acoustic velocity is low bymaking the densities different in a single crystal piezoelectricmaterial. This structure can confine the acoustic wave in the secondregion 10 b without making the bulk wave reflected.

In the first embodiment, the piezoelectric substrate 10 is made of asingle crystal piezoelectric material, and includes the first region 10a including the upper surface, and the second region 10 b located underthe first region 10 a and having a density less than that of the firstregion 10 a. The IDT 21 is located on the upper surface of thepiezoelectric substrate 10. This structure makes the energy of the bulkwave concentrate in the first region 10 a, improving the insertion lossof the acoustic wave device. The densities of the first and secondregions 10 a and 10 b can be estimated from the lithium compositionratio by X-ray diffractometry.

In addition, the velocity of the acoustic wave in the second region 10 bis greater than the velocity of the acoustic wave in the first region 10a. This structure allows the energy of the bulk wave to concentrate inthe first region 10 a.

Furthermore, when the piezoelectric substrate 10 is a lithium tantalatesubstrate or a lithium niobate substrate, the first region 10 a has acongruent composition, and the second region 10 b has a stoichiometrycomposition. This structure can make the velocity of the acoustic wavein the second region 10 b greater than that in the first region 10 a.

Located between the first region 10 a and the second region 10 b is thethird region 10 c of which the density changes from the first region 10a to the second region 10 b. This structure can inhibit the reflectionof the bulk wave due to the rapid change in density.

The thickness of the first region 10 a is preferably equal to or greaterthan the pitch λ of the electrode fingers 14 in the IDT 21. The surfaceacoustic wave energy concentrates in a region from the upper surface ofthe piezoelectric substrate 10 to the depth of approximately λ. Thus,when the thickness of the first region 10 a is less than λ, the surfaceacoustic wave attenuates. Therefore, the thickness of the first region10 a is preferably equal to or greater than the pitch λ of the electrodefingers 14 in the IDT 21. The thickness of the first region 10 a ispreferably 2λ or greater, more preferably 5λ or greater. To concentratethe energy of the bulk wave in the first region 10 a, the thickness ofthe first region 10 a is preferably 20λ or less, more preferably 10λ orless.

To concentrate the energy of the bulk wave in the first region 10 a, thethickness of the second region 10 b is preferably 10λ or greater, morepreferably 20λ or greater. To inhibit the reflection of the bulk wave,the thickness of the third region 10 c is preferably 1λ or greater, morepreferably 2λ or greater. To concentrate the energy of the bulk wave inthe first region 10 a, the thickness of the third region 10 c ispreferably 5λ or less, more preferably 10λ or less.

An exemplary case where the lithium composition (i.e., the density) isapproximately constant in each of the first region 10 a and the secondregion 10 b has been described, but the lithium composition (thedensity) may be inclined in the thickness direction in each of the firstregion 10 a and the second region 10 b. For example, it is only requiredthat the average density of the first region 10 a is greater than theaverage density of the second region 10 b.

A description will next be given of a fabrication method of the firstembodiment. FIG. 4A through FIG. 4D are cross-sectional viewsillustrating a method of fabricating the acoustic wave resonator inaccordance with the first embodiment. As illustrated in FIG. 4A, apiezoelectric substrate 10 d with a congruent composition is prepared. Alithium tantalate substrate or a lithium niobate substrate is preparedas the piezoelectric substrate 10 d.

As illustrated in FIG. 4B, the second region 10 b with a stoichiometrycomposition is formed by diffusing lithium to the upper and lowersurfaces of the piezoelectric substrate 10 d. As a method of diffusinglithium, employed is, for example, the method disclosed in PatentDocument 1. A region between the second regions 10 b becomes the firstregion 10 a with a congruent composition. Between the first region 10 aand the second region 10 b, formed is the third region 10 c of which thelithium composition gradually changes. The above processes form apiezoelectric substrate 10 e. The first region 10 a may be formed onlyon the lower surface of the upper and lower surfaces by diffusinglithium only to the lower surface of the lower and upper surfaces of thepiezoelectric substrate 10 d.

As illustrated in FIG. 4C, the upper surface of the piezoelectricsubstrate 10 is polished to expose the first region 10 a. This processforms the piezoelectric substrate 10 including the first region 10 a,the second region 10 b, and the third region 10 c. As illustrated inFIG. 4D, the metal film 12 is formed on the upper surface of thepiezoelectric substrate 10. The IDT 21 and the reflectors 22 are formedof the metal film 12. The metal film 12 is formed by, for example,evaporation and liftoff. The metal layer 12 may be formed by sputteringand etching. Then, the separation into individual chips by dicing or thelike is performed.

Second Embodiment

FIG. 5 is a cross-sectional view of an acoustic wave resonator inaccordance with a second embodiment. As illustrated in FIG. 5, the uppersurface of a support substrate 11 and the lower surface of thepiezoelectric substrate 10 are bonded together. The bonded surface ofthe piezoelectric substrate 10 and the support substrate 11 is a planesurface and flat. The support substrate 11 is, for example, aninsulating substrate such as a sapphire substrate, an alumina substrate,or a spinel substrate, or a semiconductor substrate such as a siliconsubstrate.

FIG. 6 is a graph of acoustic velocity versus depth in the piezoelectricsubstrate and the support substrate in the second embodiment. Asillustrated in FIG. 6, the acoustic velocity of the support substrate 11is greater than that in the second region 10 b. Thus, the energy of thebulk wave concentrates in the first region 10 a more than that in thefirst embodiment. Therefore, the insertion loss of the acoustic wavedevice can be further improved.

As described above, in the second embodiment, the support substrate 11is bonded under the second region 10 b, and has an acoustic velocitygreater than that in the second region 10 b. This structure can furtherimprove the insertion loss of the acoustic wave device. In addition, bymaking the linear thermal expansion coefficient of the support substrate11 less than that of the piezoelectric substrate 10, the frequencytemperature dependence of the acoustic wave device can be reduced.

FIG. 7A through FIG. 7C are cross-sectional views illustrating a methodof fabricating the acoustic wave resonator in accordance with the secondembodiment. As illustrated in FIG. 7A, the piezoelectric substrate 10 ein FIG. 4B of the first embodiment is bonded onto the support substrate11.

The example of the bonding of the piezoelectric substrate 10 e and thesupport substrate 11 will be described. The upper surface of the supportsubstrate 11 and the lower surface of the piezoelectric substrate 10 eare irradiated with the ion beam, the neutral beam, or plasma of aninert gas. This process forms an amorphous layer with a thickness of aseveral nanometers on the upper surface of the support substrate 11 andthe lower surface of the piezoelectric substrate 10 e. Dangling bondsare formed on the surface of the amorphous layer. The presence of thedangling bonds puts the upper surface of the support substrate 11 andthe lower surface of the piezoelectric substrate 10 e in an activestate. The dangling bond on the upper surface of the support substrate11 bonds to the dangling bond on the lower surface of the piezoelectricsubstrate 10 e. Thus, the support substrate 11 and the piezoelectricsubstrate 10 e are bonded together at normal temperature. The amorphouslayer is integrally formed between the bonded support substrate 11 andthe bonded piezoelectric substrate 10 e. The amorphous layer has athickness of, for example, 1 to 8 nm.

As illustrated in FIG. 7B, the upper surface of the piezoelectricsubstrate 10 e is polished so that the first region 10 a is exposed. Asillustrated in FIG. 7C, as in FIG. 4D, the IDT 21 and the reflectors 22formed of the metal film 12 are formed. Then, the lower surface of thesupport substrate 11 may be polished. Thereafter, performed is theseparation into individual chips by dicing or the like.

Third Embodiment

A third embodiment uses the acoustic wave resonator of any one of thefirst and second embodiments for a filter or a duplexer. FIG. 8A is acircuit diagram of a ladder-type filter in accordance with the thirdembodiment. As illustrated in FIG. 8A, series resonators S1 through S4are connected in series between an input terminal In and an outputterminal Out. Parallel resonators P1 through P3 are connected inparallel between the input terminal In and the output terminal Out. Atleast one of the series resonators S1 through S4 and the parallelresonators P1 through P3 may be the acoustic wave resonator of the firstor second embodiment. The number of and the connection of the seriesresonators and the parallel resonators may be appropriately designed.The acoustic wave resonator of the first or second embodiment may beused for a multimode filter.

FIG. 8B is a block diagram of a multiplexer in accordance with avariation of the third embodiment. As illustrated in FIG. 8B, a transmitfilter 80 is connected between a common terminal Ant and a transmitterminal Tx. A receive filter 82 is connected between the commonterminal Ant and a receive terminal Rx. The transmit filter 80 transmitssignals in the transmit band to the common terminal Ant among signalsinput from the transmit terminal Tx, and suppresses signals in otherbands. The receive filter 82 allows signals in the receive band amongsignals input from the common terminal Ant to pass therethrough, andsuppresses signals in other bands. At least one of the transmit filter80 or the receive filter 82 may be the filter of the third embodiment. Aduplexer has been described as a multiplexer, but at least one filter ina triplexer or a quadplexer may be the filter of the third embodiment.

Although the embodiments of the present invention have been described indetail, it is to be understood that the various change, substitutions,and alterations could be made hereto without departing from the spiritand scope of the invention.

What is claimed is:
 1. An acoustic wave device comprising: apiezoelectric substrate that is made of a single crystal piezoelectricmaterial, and includes a first region including an upper surface, and asecond region that is located under the first region and has a densityless than a density of the first region; and an IDT located on the uppersurface of the piezoelectric substrate, wherein: the piezoelectricsubstrate is a lithium tantalate substrate or a lithium niobatesubstrate, and the first region has a congruent composition, and thesecond region has a stoichiometry composition.
 2. The acoustic wavedevice according to claim 1, wherein an acoustic velocity in the secondregion is greater than an acoustic velocity in the first region.
 3. Theacoustic wave device according to claim 1, further comprising: a filterincluding the IDT.
 4. The acoustic wave device according to claim 3,further comprising: a multiplexer including the filter.
 5. The acousticwave device according to claim 1, further comprising: a supportsubstrate that is bonded under the second region and has an acousticvelocity greater than an acoustic velocity in the second region.
 6. Theacoustic wave device according to claim 1, wherein a thickness of thefirst region is equal to or greater than a pitch of electrode fingers inthe IDT.
 7. A acoustic wave device, comprising: a piezoelectricsubstrate that is made of a single crystal piezoelectric material, andincludes a first region including an upper surface, and a second regionthat is located under the first region and has a density less than adensity of the first region; and an IDT located on the upper surface ofthe piezoelectric substrate, wherein: the piezoelectric substrateincludes a third region that is located between the first region and thesecond region and of which a density changes from the first region tothe second region.
 8. The acoustic wave device according to claim 7,wherein: the piezoelectric substrate is a lithium tantalate substrate ora lithium niobate substrate, and the first region has a congruentcomposition, and the second region has a stoichiometry composition. 9.The acoustic wave device according to claim 7, wherein an acousticvelocity in the second region is greater than an acoustic velocity inthe first region.
 10. The acoustic wave device according to claim 7,further comprising: a support substrate that is bonded under the secondregion and has an acoustic velocity greater than an acoustic velocity inthe second region.
 11. The acoustic wave device according to claim 7,wherein a thickness of the first region is equal to or greater than apitch of electrode fingers in the IDT.
 12. The acoustic wave deviceaccording to claim 7, further comprising: a filter including the IDT.13. The acoustic wave device according to claim 12, further comprising:a multiplexer including the filter.